1. Field of the Invention
The present invention relates generally to the field of water treatment, and more specifically, to an elevated swale for the treatment of contaminated stormwater.
2. Description of the Related Art
Contaminated surface runoff from stormwater events is a major potential cause of water pollution in urban, residential and agricultural settings around the world. During and after rainstorms and snowstorms, storm runoff picks up a wide variety of contaminants as it flows across the surface and then into private and public waters. Runoff that flows across roads and parking lots picks up oil, grease and metals from automobile discharges; it picks up nitrate and phosphate from fertilized lawns and golf courses; it picks up organic waste, herbicides and pesticides from agricultural sites; and it picks up grit and colloidal particles from all of these locations. These contaminants have the potential to pollute receiving waters such as streams, rivers and lakes unless the runoff is treated prior to entering the receiving waters. In order to control stormwater pollution, public authorities must provide methods for intercepting and treating these contaminated flows.
Stormwater can be treated by a variety of methods, including detention ponds, constructed wetlands, infiltration basins, constructed filters, and open-channel swales. Treatment typically requires a combination of mechanical filtration (or settling) in combination with biological treatment. In general, mechanical filtration/settling removes the suspended particles, while biological treatment removes the nutrients and organic materials. (The removal of nutrients and other contaminants by bacteria and plants is commonly called “biological filtration”).
Extensive research, experimentation and monitoring have been done in the U.S. within the past decade to evaluate and improve the various methods of stormwater treatment. One method that has shown excellent efficacy is the “treatment swale.” According to the Center for Watershed Protection, the term swale refers to a “vegetated, open channel management practice designed specifically to treat and attenuate stormwater runoff for a specified water quality volume.” Conventional treatment swales can be either “wet” (in which contaminated water flows above ground surface over and through vegetation growing in the swale) or “dry” (in which contaminated water flows below ground surface through a permeable blend of sand, fine soil particles, and/or organic matter). In both the wet and dry swale structures, solid particles are mechanically filtered out, while nutrients and organics are biologically treated by beneficial, naturally occurring bacteria that colonize the surfaces of the vegetation and soil particles and break down the undesirable nutrients (such as ammonium and phosphate) and organics (such as manure and plant detritus) during their normal metabolic activity. For example, certain bacteria use ammonium as an energy source, and convert it to nitrite and then nitrate during the energy-extraction process. Other bacteria convert nitrate to harmless nitrogen gas, which then vents to the atmosphere. These same bacteria uptake phosphate and incorporate it into cellular biomass during the nitrogen conversion processes. In addition, the bacteria form sticky biofilms that tend to attract and trap particulate contaminants, such as metals that are bound to colloidal particles.
There are numerous examples in the prior art of “treatment in a box” types of remediation systems, wherein polluted water is passed through porous and permeable treatment media that are encapsulated within various types of containers. Examples of these types of systems are disclosed in Vandervelde et al. (U.S. Pat. No. 5,281,332), Towndrow (U.S. Pat. No. 6,858,142) and Kent (U.S. Patent Application Pub. No. 2008/0251448). In these and other similar examples of prior art, the containment systems are comprised of rigid exterior walls and are not designed to be fitted into channels. There is one example in the prior art (Rainer, U.S. Pat. No. 5,595,652) of a treatment structure that is designed to snugly fit into a pipe, thereby preventing by-pass of water around the structure. This device is a simple tubular container filled with pieces of sponge that expand when exposed to water. Although this device may be suitable for use in enclosed pipes of circular cross section, it is not readily adaptable for use in open channels of non-circular cross section, particularly if the channel surface is irregular. For example, the expansion of this device would tend to cause the device to “pop out” of a trapezoidal channel as the device expanded because it comprises no means of attaching the device to the channel walls.
Although conventional wet and dry treatment swales are both useful for the treatment of contaminated stormwater, each has numerous drawbacks. For example, wet swales are poorly accepted in residential settings due to the nuisance surface flows that promote noxious pests such as mosquitoes and may produce drowning hazards for children, while dry swales have the disadvantage of requiring relatively disruptive and expensive earth work. The present invention incorporates the advantages and eliminates the disadvantages of each of these prior art swale systems, while also incorporating several desirable new features that are not present in either type of existing conventional swale.